The present invention relates to a power tool, in particular a hand-operated chiseling power tool.
In the case of hand-held chiseling power tools, chiseling action is supposed to be suspended when a chisel is lifted off a workpiece. In the case of striking mechanisms that operate pneumatically, a pneumatic spring can be deactivated by means of additional ventilation openings, which are only opened if the chisel is disengaged. A striker, also called an intermediate striking device or anvil, is supposed to remain away from the ventilation openings for this purpose after an empty impact. However, this is not the case to some extent due to the rebound of the striker on the forward limit stop.
A power tool according to the invention has a striker, a guide tube in which the striker is guided along an axis, and a pneumatic chamber, which is closed by the striker, the guide tube and a valve device actuated by its own medium. A volume of the pneumatic chamber changes with the movement of the striker along the axis. The valve device actuated by its own medium has a swivelable sealing element between the striker and the guide tube. The swivelable sealing element, in the case of a movement of the striker in the impact direction, swivels into a retracted position and, in the case of a movement of the striker against the impact direction, swivels into an extended position. In the retracted position, the sealing element has a first inflow surface, defined by the projection of the sealing element onto a plane perpendicular to the axis. In the extended position, the sealing element has a second inflow surface, likewise defined as the surface of a projection of the sealing element onto the plane perpendicular to the axis. The second inflow surface is larger than the first inflow surface. In the retracted position, the radial dimension of the sealing element is less than in the extended position. The pneumatic chamber serves as a striker brake, which is controlled by the movement direction of the striker. The pneumatic chamber is closed by the valve device when the striker goes into the power tool after an empty impact for example. The pressure changing in the pneumatic chamber with the movement of the striker causes the striker to decelerate. The valve device opens the pneumatic chamber when the striker is moved in the impact direction. The brake is deactivated.
One embodiment provides that if the volume of the pneumatic chamber is increasing in the case of a movement of the striker in the impact direction, the swivelable sealing element will be swiveled into the retracted position when a pressure gradient is falling in the direction of the pneumatic chamber, and, when a pressure gradient is rising in the direction of the pneumatic chamber, will be swiveled into the extended position, and if the volume of the pneumatic chamber is decreasing in the case of a movement of the striker in the impact direction, the swivelable sealing element will be swiveled into the retracted position when a pressure gradient is rising in the direction of the pneumatic chamber and, when a pressure gradient is falling in the direction of the pneumatic chamber, will be swiveled into the extended position.
One embodiment has a further pneumatic chamber, which is closed by the striker, the guide tube and the valve device actuated by its own medium, wherein the volume of the one pneumatic chamber is increasing in the case of a movement of the striker in the impact direction and a volume of the further pneumatic chamber is decreasing in the case of a movement of the striker and wherein the pneumatic chamber and the further pneumatic chamber are connected by the valve device actuated by its own medium.
One embodiment provides that the sealing element is fastened on the striker and, in the extended position, a contact section of the sealing element touches the guide tube or alternatively the sealing element is fastened on the guide tube and, in the extended position, the contact section of the sealing element touches the striker. The touching contact section limits the swivel movement of the movable section of the sealing element. The sealing element is hereby stabilized in the extended position.
One embodiment provides that, if the volume of the pneumatic chamber is increasing in the case of a movement of the striker in the impact direction, a swivel joint of the sealing element opposite from the contact section is moved further away from the pneumatic chamber along the axis, and, if the volume of the pneumatic chamber is decreasing in the case of a movement of the striker in the impact direction, the swivel joint of the sealing element opposite from the contact section is arranged closer to the pneumatic chamber along the axis. The swivel joint may be formed by a solid-body joint.
One embodiment provides that the sealing element is fastened on the striker or the guide tube with a fastening section and a lip of the sealing element is inclined with respect to the axis, wherein, if the volume of the pneumatic chamber is increasing in the case of a movement of the striker in the impact direction, the lip is inclined away from the fastening section along the axis towards the pneumatic chamber, and, if the volume of the pneumatic chamber is decreasing in the case of a movement of the striker in the impact direction, the lip is inclined away from the fastening section along the axis away from the pneumatic chamber.
One embodiment provides that the sealing element has a V-shaped or U-shaped cross-sectional profile along the axis, wherein the cross-sectional profile is open in the direction of the pneumatic chamber, if the volume of the pneumatic chamber is increasing in the case of a movement of the striker in the impact direction, and the cross-sectional profile facing away from the pneumatic chamber is opened, if the volume of the pneumatic chamber is decreasing in the case of a movement of the striker in the impact direction.
One embodiment provides that the sealing element is asymmetric with respect to all planes perpendicular to the axis.
One embodiment has a limit stop on which the swivelable sealing element rests in the extended position and from which it is spaced apart in the retracted position. The limit stop supports the sealing element in the extended position against the forces acting on the sealing element.
One embodiment has a throttle, which connects the pneumatic chamber with an air reservoir. An effective cross-sectional area of pneumatic chamber defined by the differential of the volume of the pneumatic chamber in the impact direction is greater than a hundred times a cross-sectional area of the throttle. The striker is moved parallel to the axis, whereby a volume change of the pneumatic chamber is produced proportional to the displacement along the axis and the effective cross-sectional area. The effective cross-sectional area can be determined by the mathematical operation of differentiation in the movement or impact direction. In the case of a cylindrical guide and a cylindrical striker, the effective cross-sectional area corresponds to the largest cross-sectional area perpendicular to the axis. The ratio of the effective cross-sectional area of the pneumatic chamber to the cross-sectional area of the throttle determines a relative flow speed of the air in the throttle related to the speed of the striker. Starting at this relative flow speed the air can escape quickly enough from the pneumatic chamber without a drop in pressure with respect to the environment developing. It was recognized that an absolute speed of the air in the throttle cannot be exceeded. However, the throttle appears to block a limit value of the absolute speed. The ratio of a hundred times, preferably three-hundred times, is selected so that, in the case of a striker driven by the striking mechanism, the absolute speed of the air in the throttle is reached; in the case of a striker moved manually, the absolute speed is fallen short of considerably. As a result, the throttle blocks when the striker strikes, and opens when the striker is moved manually.
In the extended position of the swivelable sealing element, a flow channel through the valve device may have a cross-sectional area, which is less than one hundredth of the effective cross-sectional area of the pneumatic chamber. The cross-sectional area may be configured, for example, to be greater than 1/1500 or greater than 1/2000 of the effective cross-sectional area. The cross-sectional area of the closed/throttling valve may be formed in the sealing element by boreholes, notches and/or grooves running along the axis.
The following description explains the invention on the basis of exemplary embodiments and figures.